1. Field of the Invention
This invention relates to improvements in power amplifiers, and, more particularly, to improvements in push-pull transistor amplifiers that enable the highest possible output voltage swing and highest possible output current drive for power amplifiers of this type.
2. Description of the Relevant Art
Recently, increased interest has been directed towards realizing low distortion class AB power amplifiers that have low distortion and that can be realized in a monolithic integrated circuit structure.
To this end, for example, Malhi, et al., have proposed a circuit in "A Low-Voltage Micropower JFET/Bipolar Operational Amplifier", IEEE Journal of Solid-State Circuits, Vol. SC-16, No. 6, December, 1981, that has an output stage in which when current is sinked through the lower output transistor, the current through the lower output transistor becomes large, and the current through the upper output transistor, becomes nearly zero. This makes the output impedance become very large, and creates stability problems. This problem was addressed by Seevinck, et al. in a circuit described in "A Low-Distortion Output Stage with Improved Stability for Monolithic Power Amplifiers", IEEE Journal of Solid-State Circuits, Vol. 23, No. 3, June 1988, in which the output stage of the reference enables operation only as close to +V.sub.s as V.sub.s -V.sub.ce SAT -2V.sub.be and as close to -V.sub.s as -V.sub.s +V.sub.ce SAT. This circuit has a relatively high output voltage swing capability, and does not use PNP transistors.
A disadvantage of the Seevinck, et al. circuit is that since the base current of the output drive transistor and its associated current sense transistor is supplied by one of the active load transistors of the biasing control loop, if a lot of current is to be sourced from the output drive transistor, the current through the active load transistor needs to be large enough. Thus, large currents need to be supplied to the active load transistor in the bias control loop.
In the Seevinck et al circuit, additionally, large capacitors are required, making integration of all of the constituent parts of the circuit difficult.